Cellular-radio telephone systems divide a coverage area into a plurality of contiguous cells each served by a base station. In urban areas, the coverage of each cell may be only a few kilometers in diameter, cell size being reduced as the communication traffic density increases.
As the traffic density decreases the size of the cells can be increased. The protocol adopted by GSM permits mobiles to advance their timing by a maximum of 63 communication bits equivalent to approximately 35 kilometers. Once the mobile exceeds this distance it cannot be instructed to advance its timing any further. Consequently the base station receiver will eventually be unable to correlate the received mobile transmission once the timing advance becomes too great. In some GSM systems, this may be only 4-6 bits greater than the maximum timing advance (63 bits).
An option available with some communication systems including GSM is the provision of a diversity receive antenna at the base station which will receive signals from a different direction from the main beam antenna.
In particular, in some systems the diversity advantage is applied after demodulate and decoding. This is referred to as baseband soft combining and implies that two separate receiver chains are maintained in the base station from the RF input until baseband recombination.